1. Field of the Invention
This invention relates to a semiconductor light emitting device and, in particular, to a high-brightness semiconductor light emitting device with a transparent conductive film to serve as a current spreading layer.
2. Description of the Related Art
In recent years, the crystalline quality of GaN-based or AlGaInP-based semiconductors is improved since they can be grown by a MOVPE (metalorganic vapor phase epitaxy) method. Thus, a high-brightness blue, green, orange, yellow, and red light-emitting diode (herein referred to as LED) as a semiconductor light emitting device can be fabricated.
However, in order to achieve the high brightness, the current spreading property needs to be improved such that current is uniformly supplied into a chip plane of an LED. For example, an AlGaInP-based LED device is fabricated such that the current spreading layer has a thickness increased to about 5 to 10 μm. Therefore, the growth of the thick current spreading layer causes an increase in the fabrication cost of the LED device. Thus, the LED device is difficult to fabricate at low cost.
In consideration of this, a method is proposed that an ITO (indium tin oxide) or ZnO (zinc oxide) film is used as the current spreading layer to get a sufficient translucency and good current spreading characteristics (JP-A-8-83927). Further, a method is proposed that an ITO film is directly formed on a p-type cladding layer (U.S. Reissued Pat. No.35665).
When the ITO film is used as the current spreading layer, the conventional method of increasing the thickness of the semiconductor layer as the current spreading layer to about 5 to 10 μm is not necessary, and the formation of the epitaxial layer can be saved by that much. Thus, the high-brightness LED device and the epitaxial wafer for the LED device can be fabricated at low cost.
However, when the ITO film is used as a window layer, a contact resistance is generated between the semiconductor layer and the ITO film of a metal oxide, and a forward voltage disadvantageously increases. More specifically, the ITO film used as a transparent conductive film (transparent electrode) is an n-type semiconductor. On the other hand, the upper cladding layer contacting the ITO film is a p-type semiconductor. Therefore, when a forward voltage is applied to the LED, a reverse bias is established between the transparent conductive film (transparent electrode) and the p-type cladding layer. Because of this, a large voltage (i.e., increased operating voltage) has to be applied to flow current therethrough.
To solve this problem, a method is proposed that a high carrier concentration layer (=contact layer) is formed to contact the ITO film to offer a tunnel junction which allows the LED to be driven at a low voltage (e.g., Reissue Pat. No. 35665).
However, the contact layer needs to be formed as a high carrier concentration layer and a thin film since it has to offer the tunnel junction and can be a light-absorbing layer to light emitted from the active layer. Therefore, the dopant diffusion is likely to occur due to heat generated during the growth. As a result, the following two problems will be caused.
First, the dopant is diffused from the contact layer to the depth direction of the LED device. When the dopant reaches the active layer of the LED device, the dopant causes a defect in the active layer. The defect will work as a nonradiative recombination component to lower the optical output of the LED device.
Second, since a substantial carrier concentration of the high-carrier concentration contact layer lowers due to the dopant diffusion, the tunnel junction is difficult to obtain and the tunnel voltage is increased. For this reason, the drive voltage (forward voltage) of the LED device disadvantageously increases.
Zn is widely used as a p-type dopant for AlGaInP-based or AlGaAs-based compound semiconductors, but it is known that its diffusion constant is relatively large and it causes an adverse effect during heat process. Therefore, when Zn is doped into the p-type cladding layer to increase a carrier concentration thereof, the Zn is diffused into the active layer so that the characteristics of the LED device are deteriorated.
Further, when Mg, which has a smaller diffusion constant than Zn, is used as a p-type dopant of the p-type cladding layer to increase a carrier concentration thereof, and Zn, which can relatively easy offer a carrier concentration of 1×1019/cm3 and a sufficiently small contact resistivity, is used as a p-type dopant of the p-type contact layer, the diffusion of the p-type dopants, Zn and Mg causes a significant adverse effect since they are likely to be inter-diffused.
Meanwhile, there are three ways of doping the dopants, i.e., Mg only, Zn only and a combination of Zn and Mg. The amount of diffusion is increased in this order. Namely, the magnitude relation in diffusion amount is (Mg only)<(Zn only)<(a combination of Zn and Mg). Thus, in order to suppress the interdiffusion, only Zn or Mg is desirably doped. However, to dope only Zn or Mg causes the next advantages and disadvantages.
In case of doping Mg only, it is difficult to provide the contact layer with a high carrier concentration. For example, it is very difficult to offer the tunnel junction relative to the ITO film. On the other hand, the Mg diffusion from the p-type cladding layer to the active layer is very low so that an LED device with a long lifetime can be stably obtained.
In case of doping Zn only, in contrast to the case of doping Mg only, the tunnel junction relative to the ITO film can be relatively easy obtained. Namely, it is relatively easy to increase the carrier concentration of the contact layer. On the other hand, since the Zn diffusion from the p-cladding layer to the active layer is likely to occur, the lifetime of LED device becomes short as compared to the case of doping Mg only. Further, in case of doping Zn, it is difficult to obtain a high carrier concentration crystal for AlGaInP-based material as compared to Mg. Thus, a range of Zn carrier concentration to be set is limited and therefore it is difficult to obtain a high-brightness LED device.
In case of doping a combination of Zn and Mg, an LED device can be obtained suitably to some extent by using the following composition. First, Zn is used as a dopant for the contact layer to get the tunnel junction relative to the ITO film. Then, Mg is used as a dopant for a p-type semiconductor layer other than the contact layer, e.g., buffer layer and p-type cladding layer, to have the high-carrier concentration p-type semiconductor layer to offer a high-brightness LED device.
However, the combination of Zn and Mg causes the interdiffusion of Zn and Mg as described earlier. Therefore, in this case, it is necessary to suppress the degradation of the device lifetime.
On the other hand, when the contact layer is formed directly on the p-type cladding layer without forming the buffer layer and the ITO film is formed thereon (U.S. Reissued Pat. No.35665), the dopant is likely to reach the active layer due to the thin p-type cladding layer, whereby the device lifetime will be shortened. Further, due to the thin p-type cladding layer, the device is likely to be broken by damage in wire bonding.